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p0420 code

60K views 23 replies 11 participants last post by  JMadley1337 
#1 ·
Time for another question.The CEL in my 2000OBK has been coming for a few days, then off for a couple days, for about a year now. The only code that can be read by the fine folks at Auto Zone is the infamous P0420, which I assume is the rear oxygen sensor.
The car runs every bit as as it ever did, and mileage is no different, but I'm wondering: are there long term effects of not changing this expensive little doo-dad? Will it plug my cat, or somehow whack out the computer?
Treeman in Houston
 
#2 ·
No its the front sensor, not the rear. rears outlast the front by miles. Aside from wasting gas as time go buys, and taking a chance on hurting your cat, and hurting performance as time go buy, and maybe fouling plugs, its up to you.

The o2 sensor is the heart (er nose) of the FI and emissions system. Replace it.

Get it at an on line OE supplier (1st subaru etc).


nipper
 
#3 ·
Sorry Nipper but you're wrong on this one. The PO420 regards the rear O2 sensor and the difference in temp drop across the catalyst. Basically, when you get the code it means the cats aren't doing their job, or the sensor has fouled.

If there are exhaust modifications you can get that code, or outdoor temps being too cold...

The front O2 sensor is 99.9% of the brains when it comes to running the engine.

Brian
 
#4 ·
bheinz57 said:
Sorry Nipper but you're wrong on this one. The PO420 regards the rear O2 sensor and the difference in temp drop across the catalyst. Basically, when you get the code it means the cats aren't doing their job, or the sensor has fouled.

If there are exhaust modifications you can get that code, or outdoor temps being too cold...

The front O2 sensor is 99.9% of the brains when it comes to running the engine.

Brian
The exact code does not specify either O2 sensor, front or rear, just catalyst effeciency. It does this by comparing the data from the front o2 sensor and the rear O2 sensro. If the front O2 sensor is original and over 80,000 miles on it, odds are its the front one. The front one takes more of a beating then the rear, unless the rear gets hit by something...


which is how i killed mine :)


Considering you said its a fouled sensor, sort of seems that you agreed with me.

:15:

nipper
 
#5 ·
^Well personally I have never heard of a front O2 sensor being replaced due to that code.

I did just find basic info on the PO420:


My check engine light came on and my mechanic said the Catalytic Converter was the problem. How can I be sure?

This has been a tricky problem for most mechanics and exhaust shops, When diagnosing the problem with a scan tool. The code that comes up for an inefficient Catalytic Converter is a PO420. This is an indication that the Catalytic Converter is not doing its job properly but we have found that this code could also occur if the Oxygen Sensor (O2 Sensor) or sensors become faulty or lazy, We have also found that on the Jeep Cherokee, Subaru Outback, Ford Explorer and Honda Accord, the factory tolerances that set off the check engine light are very tight and an engine that has carbon buildup in the combustion chamber will bring up the cylinder head temperature which in turn brings up the Oxides of Nitrogen (NOX) and will set the light off. In these cases we recommend a product on the market that has been working well to cut down on the carbon build up and will reduce the dreaded check engine light and PO420 code. The product is called BG44K- Total Deposit Control Fuel Additive and can be purchased from us at our product link. A good rule of thumb here is to make sure all components that we have mentioned here have been checked before the Catalytic Converter is replaced. There is nothing worse than spending hard earned money on a Catalytic Converter and find out that the check engine light problem still exists.

A decent quick read that I found.

Brian
 
#6 ·
I have the same problem. I went to my mechanic yesterday, he pulled codes, p0304 (cyl4 misfire) and p0420 (famous exhaust code). So I am not worrying about p0304 but p0420 could get pricey if not dealt with properly. He read my o2 sensors activity, first one was reading 1.500 and the second one was 0.650. I know it's normal for the first one to read higher then the second one but does anyone know exact numbers that are acceptable?

car is the 02 impreza outback sport.

thanks!
 
#12 ·
code P0420

Ive been researching and working on this for 2 months. I need help. Front sensor is reading 1.5v and rear is .750v. Everything I read says range from .1-.9, if this is true them it is running extremely rich. At wide open throttle reading go to; front sensor reads .300 and rear reads .600. Tried some test like creating vaccuum leak but no change. Could brand new sensor be faulty?
 
#13 ·
Ive been researching and working on this for 2 months. I need help. Front sensor is reading 1.5v and rear is .750v. Everything I read says range from .1-.9, if this is true them it is running extremely rich. At wide open throttle reading go to; front sensor reads .300 and rear reads .600. Tried some test like creating vaccuum leak but no change. Could brand new sensor be faulty?
Sometimes people just over think things. How many miles are on the car? What year is it?

1998 Front 02 sensor at idle 0-0.9 shield is zero.

rear o2 sensor 0-0.9volts.

You may be reading rich because the O2 sensor is bad. The O2 is the nose of the fuel injection once the car warms up and goes into closed loop.

What is the CEL light code? Can't go anywhere without that code as a tool.
 
#18 ·
The front 02 and then the front cat are first to go I sorted that out on my old 2001.

Replaced the 02 front - still got the error - repaced the rear still got the error. Replaced the front cat error went away - for kicks I put the old rear 02 back in and no error - put the front o2 back in and got the error.

In my case a crack right at the Y pipe led to the demise of the cat and front o2 sensor. Road grime and rain water not so good for the cats.

ALWAYS start with the front one first and with the cheap parts the o2 sensors. If the cat needs replacing find a local exhaust shop I was in and out in an hour and a $150 lighter in the pocket vs the $1300 Subaru wanted.
 
#20 ·
Cheap O2 sensor for the front is the firt mistake. Front one always need to be from subaru. This has been covered many times here.

If you have a good O2 sensor rear, dump the elimintor. It is a peice of junk and a cheat.

readings at sped are invalid. My numbers come from the facotry service manual and are at idle.

Find the miss. If the wires have not been replaced, i would start there by moving the wires around and see if the miss follows it.

retake the data.
 
#21 ·
at 52 mph im getting, front 1.526 and rear .510. if my rear was reading normal range i would suspect front sensor to be bad. they are both high. Replaced plugs and wires. I found a good article about o2 sensors. Its says for rich readings to check the following. Fuel pressure, leaky injector, malfuctioning canister purge valve, low map sensor voltage, false tps signal. Im going to rule them out before buying another o2 sensor. Ill let you know.
 
#22 ·
I just found that my 2000 OBW has an open recall for the front (bank 1) 02 sensor (and also a recall fro rear suspension that was never done) through registering at mysubaru.com. I also read somewhere about the ECU needing 'reflashed' because the engine runs too rich and fries these sensors. I'm waiting to hear from a local dealer for an appointment to get this replaced.
 
#23 ·
I had the infamous P0420 code on my 2002 Outback (bought used in 2009 with 90,000 miles). Started in 2012 with about 130,000 and was intermittent for about a year, going off for a few months then coming on for a few weeks until in late 2013 the CEL was on all the time. Luckily I was able to pass NY State emissions before the light was on full time. Subaru told me the cat needed to be replaced and they'd be happy to do it for $1,700. Another local shop told me they'd put a Chinese cat in for $650 but it would probably not last longer than a year.

I decided, after reading everyone else's horror stories here on Outback.org to replace both O2 sensors myself. $100 for the rear sensor, $60 for the front one. I also bought the claw foot tool. However, since I really hate getting underneath a car, my brother told me about this guy he knows down in the rough part of town who has a repair shop. Went down there with my brother and the guy asked, "did you spray them with WD40 yet?", I said, "yes, I did last night", he said "80 bucks to change both". I said "ok great", but I really wanted to jump for joy. The guy later said O2 sensors get "sluggish" and then fail and will throw a P0420 code. He said it's usually the rear sensor that should be changed first. I have 152,780 miles on my OB now.

Came back to the shop an hour later, the job was done. CEL has been off for over 300 miles already, passed emissions. I think $240 is a lot better than $1700 or $650, don't you? Sure the CEL could come back on at anytime, but these dealers and shops don't care about you, they just see you as an ATM machine. This website is one heck of a resource for DIY or finding out options to really expensive repairs that might not be necessary.
 
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